PURPOSE: To provide a nitrogen removing apparatus which can reduce the retention time in a wastewater reaction tank and is satisfactory in terms of durability and coats.

CONSTITUTION: In this biological nitrogen removing apparatus in which a denitrification tank 1 and a nitrification tank 2 are arranged, porous carriers 15 with communicating pares which are made mainly from polyvinyl formal are put into the nitrification tank 2. The pore size of the carriers 15 is less than 100μm, the shape of the carriers 15 is cylindrical, hollow-cylindrical, or cubic. The hydrophilic property of the polyvinyl formal helps water to be treated enter the pores and also enlarges the surface area of the carriers. Also, the mass production of the carrier is easy.